The present invention relates to the production of purified crystals from a polymorphic compound containing impurities.
One common method of purifying a compound is to crystallize the compound in a solution. Methods of crystallization typically involve controlling macroscopic external variables such as evaporating solvent to create supersaturation or adjusting the temperature of the solvent to affect solubility. These crystallization methods are generally directed to achieving maximum solids recovery and/or purification without any regard to the size or shape of the crystals.
Therefore, there is a need for a crystallization process that can effectively control or influence the ratio of crystal growth to formation of new crystals at low energy costs.
The present invention is based on the discovery that some chemical compounds having multiple crystal forms (i.e., polymorphic compounds) have unexpectedly high dissolution rates and have unexpectedly stable supersaturation capacities under appropriate conditions that can be rapidly relieved by the introduction of crystal surfaces of the polymorphic compound in a first crystal structure to produce relatively large crystals of the compound at high rates of crystal growth. Some of these compounds occur naturally in deposits, such as evaporite deposits. The resulting crystals which are larger than insoluble impurities can be readily separated from insoluble impurities on a size separation basis.
A common procedure for processing such ores is to dissolve the mineral to be recovered and separate the insoluble impurities by clarification methods, such as thickening and/or filtration. The water added in the process must be removed, generally by expensive methods, such as evaporation.
More particularly, the process of the present invention is for producing a polymorphic compound in a first crystal structure from a feedstream which includes in a second crystal structure and insoluble impurities. The process includes adding the feedstream to a saturated brine of the compound under conditions to create supersaturation of at least about 5 g/l. The process further includes processing within parameters that preferentially relieve the supersaturation by rapid growth of existing crystals of the compound in the first crystal structure rather than by nucleation. In this manner, the particle size distribution of crystals is controlled to achieve a desired distribution of crystal size product which is coarse enough that it can be separated from the insoluble impurities on a size basis. The crystals of the compound in the first crystal structure produced by the process are recovered from the saturated brine solution.
The process can include the use of a high feed rate of at least about 100 grams of feedstream per minute for each liter of solution in the crystallizer. The process can also include relieving the supersaturation preferentially by rapid growth of existing crystals of the compound in the first crystal structure over nucleation by adding seed crystals of the compound in the first crystal structure to the saturated brine solution of the compound. Such seed crystals can be produced by removing crystals of the compound in the first crystal structure from the brine solution and sizing the removed crystals to produce a seed crystal size fraction for reintroduction to the brine solution, by producing the seed crystals separately, and/or by grinding or partial dissolution of part of the product. In a preferred embodiment, the particle size of the feedstream is less than about 150 mesh and the particle size of the seed crystals is from about 100 mesh to about 150 mesh.
Relief of supersaturation preferentially by rapid growth of existing crystals of the compound in the first crystal structure over nucleation can alternatively be achieved by a variety of methods. Such methods can include maintaining a solids content of at least about 40% in the crystallizer, agitating the brine solution at an agitation index of at least about 4, periodically lowering the temperature of the brine solution by at least about 5xc2x0 C., or pausing feedstream addition at least about 60% of the time of crystallization.